The thawing of Robert De Niro has been as astonishing to witness as the shattering of a glacier cliff.

It started on the promotional trail for Silver Linings Playbook, during a Katie Couric interview in which De Niro broke down – or, more accurately, the audience waited respectfully while the actor arm-wrestled his welling emotions to the ground, holding up a hand apologetically the way you might for waiting traffic. Couric didn’t have the heart to hit him with a follow-up.

The actor teared up again last week when talking to Out magazine about his artist father, who was gay – as a new documentary reveals.

“I get emotional,” he said. “I don’t know why.”

As if there might be something at all unusual about a son getting choked up about his about the struggles of his dead dad.

But then this is De Niro, whose character in Michael Mann’s Heat, a super-thief named Neil McCauley, is first introduced perusing a book on titanium stress fractures and whose powers of self-abnegation are summed up in his advice to never get too attached to anything you are “not willing to walk out on in 30 seconds flat if you feel the heat around the corner”.

There were always signs of the emotion buried several fathoms below the tungsten-tough surface of his performances. Martin Scorsese, for example, shot 19 takes of the climax to Raging Bull, in which La Motta delivers the “I could have been a contender…” speech from On The Waterfront to his dressing-room mirror. De Niro picked out take 13, the more emotional. Scorsese liked a flatter one.

So they watched both, back to back, but failed to change each other’s mind. Scorsese chose his.

“I still think the one I have in is best,” said the director.

“All right,” said De Niro. “Let it go.”

Was take 13 better? We’ll never know. We have Scorsese’s Raging Bull, not De Niro’s, another of their grand excavations of men hollowed out by their rages and resentments, to set alongside Travis Bickle and Rupert Pupkin – psychotically thin-skinned, hazy with delusion, collecting injuries and injustices the way Catholics count rosaries. And yet by his early 30s, De Niro knew great success playing such men, and could count two Oscars on his mantelpiece. The big mystery with him has always been: where did that pressure cooker of alienation and resentment come from? What set the bull raging?

At least a partial answer is glimpsed in Remembering The Artist Robert De Niro Sr. Directed by Perri Peltz, this new film tells two stories. The first is that of the father, craggily good-looking just like his son, who in the 1940s and 50s found some fame as an artist.

De Niro Sr was one of Hans Hoffman’s favorite students, was exhibited by Peggy Guggenheim and acclaimed by Art News alongside abstract expressionists like Jackson Pollock and Mark Rothko. Less auspiciously, his work also showed the lingering influence of post-impressionists like Pierre Bonnard and Henri Matisse, his “luxuriousness, calm and voluptuousness” standing out like a sore thumb at a time when American painting was gutting itself of just those qualities.

De Niro Sr's work also showed the lingering influence of post-impressionists like Pierre Bonnard and Henri Matisse. Photograph: Startraks Photo /REX

“'Too French,' they would say, ‘Not American enough,'” recalls one contemporary, who subsequently saw his friend sink into neglect, sucked under by jealousy, resentment and bouts of depression, as American art fell for the bright acrylic glare of pop art.

“I feel I hardly have the courage at this moment to wash my brushes, which have been standing in turpentine,” goes a journal entry, read in voiceover by his son.

De Niro uses the same flat, telephone-book effect he used to read Travis Bickle’s journals in Taxi Driver: “June 8th. My life has taken another turn again. The days can go on with regularity over and over, one day indistinguishable from the next. A long continuous chain …”

You thought you were in this alone? Travis Bickle, meet Robert De Niro Sr, another of God’s lonely men, watching the world leave him behind.

The film's second story, detailing the effect of all this on the son, is more sketchily told – one reason, perhaps, that the doc comes in at a trim 40 minutes. De Niro's parents separated when he was two and divorced when he was nine or 10, his father a fleeting presence through most of his childhood, sometimes glimpsed tantalisingly on the street.

“I’d run into him, or I’d see him on his bike,” says Bobby – a rather wrenching image, with its own tight-lipped heartbreak, although it remains unexplored. De Niro is one of the most clam-like of interviewees, but I would have liked the interviewer to press more on his memories of the darker figure he remembers from his teens, “rambling” and “ranting” about the injustices of the art world. On one occasion, De Niro and his father hauled some of his paintings around Paris, on an ill-advised and luckless sales mission, before the son planted his father on the return flight to New York.

Part of the reason for De Niro Sr’s decline was fashion, but the simplest explanation for this lack of evolution may be to do with his sexuality, his heroically painful, Cheeveresque battle with which is recorded in his journals.

“If God doesn't want me to do be a homosexual, about which I have so much guilt, he will find whom I will love and who will love me,” he writes. Later, he records a frustrated trawl of the streets, “looking for a gallery, or a lover, either for that matter” – a telling conflation which suggests how much the feelings of being alone and unloved in one sphere buttressed and reinforced the sense of it in another. In a manner eerily familiar to his son’s later acting work, De Niro Sr was in love with feeling unloved.

How much Robert De Niro’s exposure to his father’s rages and resentments bled into his later performances, and how much of it was the son exploring instincts that had doomed the father, is perhaps too impertinent a speculation. But the son achieved fame playing the painfully un-famous – which is to say not just ordinary men, but the Bickles and Rupert Pupkins of this world, who feel their lack of acclaim like an injustice in their gut.

The documentary’s most heartbreaking moment comes from a journal entry in which the father records seeing his son newly tan, for a movie role.

“I wanted to run my fingers through his hair, but I hardly think he would have appreciated it,” reads Bobby now, clearly willing to trade everything for that fleeting missed contact, but making do with this recreation of it instead.

The documentary is part festschrift, part expiation, fuelled by filial regret at things left unsaid, connections missed, as well as the desire to right wrongs and honor his father’s legacy. On one level, you wonder if De Niro hasn't spent the best part of his career already doing so.

• Remembering The Artist Robert De Niro Sr airs in the US at 9pm on HBO, 9 June.